Yes, you can eliminate possibilities by running an investigation, but in the cases where the critical witnesses are crispy bites and any suspect phone is a puddle of dried goop there's not a lot you can do to establish if it was in use or not.
[edit] In other words, the only conclusions you can really expect to get from such an investigation are "no mobile was in use" and "unknown".
Seeing as petrol stations all over the UK have warning signs about mobile use, I'm going to assume there is a reason for them to be there that isn't "some spam told us they were bad".
I don't see a poorly conducted trial on a limited set of conditions by a television program with an "appeal to the middle layer" name to be anything approaching conclusive proof, and I think lives are far too precious to be casually informing people that they should potentially put themselves and others in a great deal of danger.